In communication devices where a transmit path and a receive path share a same antenna, an intermediate device, e.g., a duplexer, may be provided to connect the transmit path and the receive path to the antenna. For example, a duplexer may separate different frequency bands used for transmission and reception of radio frequency (RF) signals by means of frequency-selective filter elements. For example, a first frequency band may be used by the transmission path for transmitting RF signals, whereas a second frequency band may be used by the reception path for receiving RF signals. A distance between a frequency band for transmission and a frequency band for reception is called “duplex distance”. For example, a duplex distance may be 30 MHz for Long Term Evolution (LTE) frequency band 17.
A duplexer should provide an adequate rejection of signal components related to transmission within a frequency band for reception. However, when a strong transmission signal, e.g., a transmission signal with great power, is provided to the duplexer, crosstalk to the frequency band used for signal reception may occur. Thus, an undesired crosstalk component may be caused in a receive signal and yield significant transmit power in the receive signal. Accordingly, a signal containing a desired receive signal component and the undesired crosstalk component related to a baseband transmit signal may be provided to a subsequent receiver. However, there may also be other processes causing undesired components in a receive signal provided to a receiver.
A mixer may be used in a receiver for down-mixing a received radio frequency signal to a baseband receive signal. A mixer may in general have a non-linear transfer function. That is, the relation between a signal input to the mixer and a signal output by the mixer is not linear. The linearity of a mixer or any other non-linear element may in general be classified using the n-th order Input Intercept Point (IIP). For a direct conversion receiver or a low intermediate frequency (IF) receiver, the second order Input Intercept Point (IIP2) of a receiver's mixer may have great influence on the receiver's performance. The IIP2 may be determined graphically by plotting an output power of a fundamental signal input in the mixer versus the input power of the fundamental signal and by plotting a same curve for a second order distortion component caused by the non-linearity of the mixer. The curve of the fundamental signal and the curve of the second order distortion component are extrapolated and the point where the extrapolated curves intersect each other is the IIP2. The IIP2 may alternatively be calculated. A higher input power associated to the IIP2 may correspond to a better linearity of the mixer since the input power at which the output power of the desirable signal and the output power of the distortion are equal is higher.
InterModulation Distortions (IMD) may be generated by the mixer receiving a signal having different frequency components. For example, unwanted signal components may be present within a baseband receive signal generated by a mixer resulting from the non-linearity of the same. Referring to the above example, signal components having a frequency related to the sum or the difference of frequencies of the desired receive signal component and the undesired crosstalk component input to the mixer may be present in the baseband receive signal generated by a mixer. In the above example, the generated undesired signal components are referred to as second order IMD components. The second order IMD components may lower a Signal-to-Noise Ratio (SNR) of the baseband receive signal, especially near sensitivity power levels of the receiver.
For reducing the second order IMD components, an offline calibration may be performed in order to enhance the linearity of a mixer. For example, the IIP2 of the mixer may be improved, e.g., shifted to higher input signal powers by the calibration. The calibration may, e.g., be performed by a manufacturer before distribution of the equipment or during stand-by operation of the equipment. For calibration, a transmission signal at a transmit carrier frequency may be provided via a closed loop calibration path to an auxiliary Low Noise Amplifier (LNA) to which the receiver is connected to. Thus, a transceiver internal test signal may be provided to the receiver which is, e.g., representative of the undesired transmit component caused by the duplexer at the presence of a strong transmission signal. As a first alternative, an external calibration signal may be provided to the auxiliary LNA which represents the undesired crosstalk component. As a second alternative, the external calibration signal may be provided to the regular LNA. However, further additional RF signal generation equipment is needed for the second alternative. Second order IMD components generated by the mixer of the receiver may be measured in the baseband receive signal provided by the mixer. A bias voltage applied to the mixer may be varied in order to minimize the second order IMD components in the baseband receive signal, which is also called mixer-tuning. However, the characteristics of the auxiliary LNA may be different from those of a regular LNA used in the receive path during normal operation for amplifying a received signal provided by the duplexer. Therefore, the bias parameters determined using the auxiliary LNA cannot be used with the regular LNA directly. Further elaborate adaption of the bias parameters is necessary, which may lead to non-optimal bias parameters for the mixer during normal operation. Moreover, calibration is merely possible at a manufacturer side or during stand-by operation. Also, the auxiliary LNA, the auxiliary receive path, the RF test signal generation and other components for the above closed loop calibration require space on a chip which includes the receive path. Hence, there may be a desire for improved reduction of distortion components in a baseband receive signal.